Yesterday Trump tweeted that the U.S. will “impose large sanctions” on Turkey unless it releases pastor Andrew Brunson, who’s accused of involvement in a coup attempt.
Seems clear enough...except that this sudden concern over some pastor came less than a single day after this happened.

Turkish Foreign Minister Mevlut Cavusoglu on Tuesday ruled out his country's compliance with U.S. sanctions on Iran, a move that threatens to exacerbate tensions between the NATO allies.

"We have told them we will not join these sanctions," said Cavusoglu, referring to a meeting last Friday with senior U.S. officials in Ankara. "While we are explaining why we will not obey these sanctions, we have also expressed that we do not find these U.S. sanctions appropriate."

Ankara strongly opposes U.S. President Donald Trump's decision to impose sanctions after pulling out of an international agreement with Iran on its nuclear energy program. Stringent sanctions are to start taking effect at the end of August, with measures against Iranian energy exports beginning in November.

I'm totally sure that the sanctions threat have nothing to do with Turkey defying our Iranian sanctions. /s
The Turkish state-controlled Halkbank was convicted some months ago for violations of previous U.S. sanctions on Iran.

“Trump’s tweet on Brunson shows that there are weaknesses in this strategy.”
- Sinan Ulgen, a former Turkish diplomat

It's rare that you see this sort of misdirection be so obvious.

Washington says no to any waivers for countries trading with Iran, which puts it on a collision course with Ankara.

Washington was already on a collision course with Ankara over Russian weapon systems.

A US defense bill would bar delivery of the F-35 Joint Strike Fighter to Turkey until the US government provides an assessment of the relations between Washington and Ankara — a move that comes over the objections of Defense Secretary Jim Mattis and underscores growing tensions between Turkey and its NATO partners.

The conflict with Turkey — which fields NATO's second-largest army and hosts important NATO infrastructure — stems largely from its decision to buy the Russia-made S-400 air-defense system, one of the most advanced systems of its kind on the market.

What is ironic is that the boondoggle that is the F-35 is partly made in Turkey.

No guidelines exist for how Turkey or any other member could be ejected from NATO.
It's unprecedented for Washington to sanction a NATO ally, and even the act of applying those sanctions comes at a cost over and above the threat to NATO unity.

“If Turkey becomes less powerful militarily, the resources that Russia allocates to check Turkey could be sent elsewhere, simple as that,” he added.

The "weaknesses in this strategy" is on display in more than just the fact we are using the excuse of the pastor to apply sanctions, rather than just calling it for what it is.

Another sign of weakness is the fact that Turkey is being singled out. India is going to get a free pass on the Russian arms sanctions.
India, having a much larger economy, simply called Washington's bluff on Russian arms purchases, and they appear to be positioning themselves to do the same with Iran sanctions.

Pradhan, however, did not give a direct reply to the question whether the US has asked some countries including India to wind-up their business with Iran by November 4.
"India has taken note of the re-imposition of US Sanctions on Iran in wake of the US pullout from the Joint Comprehensive Plan of Action on May 8, 2018," he said.
...
India pays Iran in euros using European banking channels.

During the first round of sanctions in 2012 when European Union joined the US in imposing financial restrictions, India initially used a Turkish bank to pay Iran for the oil it bought but beginning February 2013 paid nearly half of the oil import bill in rupees while keeping the remainder pending till the opening of payment routes.

Germany is encouraging India to defy the Iranian sanctions, while Germany itself also got a free pass regarding Russian energy sanctions.

Also absent from both bills is the specific targeting of Gazprom’s Nord Stream II.

With so many loopholes developing in our sanctions, Washington is broadcasting weakness, not strength. That's what makes the sanctions against Turkey so critical, and so dangerous.

Turkey however remains a crucial test case that the wider world will be watching to see who will ultimately win a second party sanctions showdown – the US or a nation which continues to do business with Russia in spite of threats of sanctions.

Can Turkey withstand the sanctions? If they can then America's sanction power will be no more.
Since Washington doesn't do diplomacy anymore, and if economic sanctions no longer work, then all we have left is dropping bombs. That doesn't work well with allies.

Then there is the danger of Turkey NOT being able to withstand the sanctions, but collapsing instead. A NATO ally descending into economic chaos is not in our interests either.
So sanctioning Turkey is a fine line to walk.

Finally, there is a danger from throwing around too many sanctions against too many countries. President Putin explains.

“As for our American partners and the restrictions they impose involving the dollar,” he added, “I think that is a major strategic mistake because they’re undermining confidence in the dollar as a reserve currency.”
"Regarding our American partners placing limitations, including those on dollar transactions, I believe is a big strategic mistake. By doing so, they are undermining the trust in the dollar as a reserve currency."

The whole idea of a reserve currency is reliability.
If there is a constant threat to access, then it isn't reliable.

“we must be conscious of the risk that overuse of sanctions could undermine our leadership position within the global economy, and the effectiveness of our sanctions themselves”.
- former US Treasury secretary Jack Lew, 2016